The present invention relates to an apparatus for restoring modulation data and error correcting/coding data for use in a digital transmission system, and more particularly, to a data restoration apparatus which further improves reliability of block deinterleaving with respect to data which is modulated using multi level/phase modulation and is error-corrected.
In a general digital communication system, a transmission signal from a transmission end is received at the receiver in a data form different from the original signal due to noise caused by poor transmission conditions of the channel. A error correction code is used for correcting an error due to such a noise, for which a convolutional encoder is employed. Since a convolutional encoder is resistive to random error and is susceptible to burst error, the encoder is generally used with an interleaver which disperses burst error added on the channel in a random manner. FIG. 1 shows an example of a conventional digital communication system which employs the convolutional encoder and the interleaver, a transmission portion 4 includes a convolutional encoder 1, an interleaver 2 and a modulator 3, and a receiving portion 8 includes a demodulator 7, deinterleaver 6 and a convolutional decoder 5. Convolutional encoder 1 convolution-codes data from the external information source and transmits the convolutional-coded data to interleaver 2, which interleaves the received data and supplies the data randomized by interleaving to modulator 3. Modulator 3 frequency-modulates the received data to transmit the modulated data to the channel. Demodulator 7 in the receiving portion 8 demodulates the modulated data which is transmitted through the channel and outputs the demodulated data to deinterleaver 6. Deinterleaver 6 rearranges and outputs the demodulated data in an order similar to that prior to interleaving. The rearranged data in deinterleaver 6 is decoded in convolutional decoder 5 and is restored to its original data form.
To improve error correction efficiency with respect to burst error, a receiver in an enhanced digital communication system is shown in FIG. 2. This system employs a PSN deinterleaving method using pseudo random numbers and a block deinterleaving method.
FIG. 2 is a data format showing a restoring apparatus for multi level/phase modulation data. This shows a series of processes for the case when data at the transmission end is interleaved, RS coded, Trellis coded, modulated and then transmitted to a receiver. At the receiver, the received data is demodulated and restored to its original data form.
In FIG. 2, a demodulator 10 demodulates the received modulated data. I-axis data (I) which is in-phase data demodulated in a demodulator and Q-axis data (Q) which is quadrature-phase data demodulated in a demodulator are supplied to a deinterleaver 13. Since the respective I-axis and Q-axis data have been interleaved, deinterleaver 13 deinterleaves the respective I-axis and Q-axis data based on a predetermined sync position and outputs the deinterleaved data to a Trellis decoder 14. Trellis decoder 14 discriminates whether or not the Trellis-decoded data is synchronized. A Viterbi algorithm is used for such sync discrimination. Trellis decoder 14 repetitively performs a process of selecting a path, where a Hamming distance is shortest among the paths to which the data is input in a Trellis diagram, to find a path having the minimum Hamming distance entirely. If a cumulative distance of the path is larger than a predetermined reference value, Trellis decoder 14 indicates that the sync does not match and generates a predetermined out-of-sync signal S.sub.1. The predetermined reference value is determined by a designer and is experimentally obtained. Out-of-sync signal S.sub.1 at the output of Trellis decoder 14 is supplied to a first sync detector 15 and a second sync detector 17.
First sync detector 15 outputs a predetermined control signal S.sub.2 which is supplied to deinterleaver 13 whenever out-of-sync signal S.sub.1 is generated. The control signal S.sub.2 causes adjustment of the sync position for a deinterleaving operation of deinterleaver 13.
Deinterleaver 13 adjusts the sync position whenever the sync adjustment control signal S.sub.2 is applied thereto, and performs a deinterleaving operation at the new sync position. The data deinterleaved according to the new sync position is supplied to Trellis decoder 14. Trellis decoder 14 Trellis-decodes the input data based on the new sync position and determines if the sync is matched. Trellis decoder 14 finds a sync from the input data by repetitively performing sync detection processes.
If a sync condition is detected after Trellis decoding, the data decoded in Trellis decoder 14 is supplied to a block deinterleaver 16. Block deinterleaver 16 deinterleaves, in units of a block, the data which has been interleaved in a modulator at the transmission end. A second sync detector 17 detects a sync condition of the block data which is deinterleaved in block deinterleaver 16. Second sync detector 17 is disabled by the out-of-sync signal S.sub.1 from Trellis decoder 14. The block data at the output of block deinterleaver 16 is supplied to a Reed Solomon (RS) decoder 18. RS decoder 18 RS-decodes the data which has been RS-coded at the transmission end to reinforce an error correction function for the transmission data. Here, RS decoder 18 decodes the block data while matching a block sync signal which is supplied from second sync detector 17.
Such a conventional restoring apparatus of the multi level/phase modulation data does not restore exactly the original information data when a phase error occurs in the demodulated I- and Q-channel data.
In order to solve this problem, a conventional method and apparatus for restoring multi level/phase modulated data is disclosed in U.S. Pat. No. 5,398,260 which issued on Mar. 14, 1995. This patent mentions technology capable of finding sync data in a demodulator even though a phase error of .+-.90.degree. is generated.
However, in a case that sync signal data for block deinterleaving which is error-corrected by the system disclosed in the above patent is different from a sync signal at the transmission end, that is, in a case that a sync signal is not completely due to added error there has a drawback that a good operation of block deinterleaving cannot be performed.